1. Field of industrial utility
This invention relates to a method of manufacturing semiconductor devices, particularly relates to a method of trench isolation.
2. Prior arts
An isolation region has incessantly been reduced in size with the miniaturization of a semiconductor device. In this trend, the trench isolation was recently proposed and has been under development. In the trench isolation, as shown in FIG. 2, trenches 14 are formed to serve as isolation regions 13 in order to electrically isolate active areas 12 in which devices such as transistors are formed. The device isolation capacity increases with the increase of the depth of the trench 14. In order to improve the device isolation capacity and to prevent the inversion of polarity, the impurity concentration of the semiconductor substrate at the device isolation region 13 is generally made higher than that of the active area. This region 15 is called a field dope region. Generally, the field dope region 15 is formed by ion implantation after trenches have been provided. However, when a trench is deep, a part of the trench is not implanted with ions. As shown in FIG. 3, because the incident angle of ions for ion implantation is adjusted at 7.degree. in order to prevent the channeling phenomenon, ions are consequently blocked by a masking resist 23 during ion implantation, which phenomenon is called the shadowing effect, and a part 25 of the trench is not implanted with ions.
The rotating ion implantation method is used in order to prevent the shadowing effect. However this method requires a complex apparatus, and the control of the apparatus is extremely difficult. There is another method called the "zero degree ion implantation method", in which ions are implanted vertically relative to the surface of the semiconductor substrate. However the drawback of this method is that ions are difficult to be implanted in the walls of the trench.
Besides the ion-implantation methods, the diffusion method is used, in which a film containing an impurity is formed on the walls of a trench after the trench has been formed, and the impurity is diffused from the film into the walls of the trench. However this method requires complicated steps. Further, since the amount of the impurity required for field doping is at most 10.sup.16 -10.sup.18 /cm.sup.3, it is difficult to control the impurity concentration at this low level by the diffusion method.
As described in the foregoing, it is extremely difficult for the conventional trench isolation method to uniformly dope the walls of a trench with a desired impurity concentration, and therefore it is almost impossible to obtain stable device isolation characteristics.